A large body of evidence indicates that personality traits show high rank-order stability and substantial mean-level changes across the lifespan. However, the majority of longitudinal research on personality development has relied on repeated assessments of self-reports, providing a narrow empirical base from which to draw conclusions and develop theory. Here, we (1) tested whether self- and informant-reports provided by couples show similar patterns of rank-order stability and mean-level change and (2) assessed self–other agreement in personality development. We charted the Big Five personality trajectories of 255 couples ( N = 510; M age = 27.01 years) who provided both self- and partner-reports at four assessments across 1.5 years. Results indicated similar rank-order stabilities in self- and partner-report data. Latent growth curve models indicated no significant differences between self- and partner-reported personality trajectories, with exceptions to extraversion and agreeableness. We further found strong cross-sectional agreement across all Big Five traits and assessment waves as well as moderate self–other agreement in personality change in emotional stability and agreeableness. These findings highlight the relevance of multi-method assessments in personality development, while providing information about personality stability and change. Discussion focuses on the theoretical implications and future directions for multi-method assessments in longitudinal personality research.